This application claims benefit of Japanese Patent Application No. 2000-076402 filed on Mar. 14, 2000, the contents of which are incorporated by the reference.
The present invention relates to foldable portable telephone sets and, more particularly, to systems for and methods of call tone output in foldable portable telephone sets.
The foldable portable telephone set has a buzzer provided on its outer side in the folded state for outputting a call tone at the time of a call arrival.
Although buzzers have heretofore been used as call tone generator for portable telephone sets, recently they have been increasingly replaced with loudspeakers as disclosed in Japanese Patent Laid-Open No. 1-85454. This trend is thought to be due to unexpectedly great popularity of the portable telephone sets giving rise to measures against noise given by them to nearby people. More specifically, the loudspeaker usually has narrower sound directivity than the sound directivity of the buzzer, and it thus enables directing the call tone at the time of a call arrival to the user and not to nearby people.
The loudspeaker, however, has a problem that it is driven by a loudspeaker drive amplifier (with an input impedance of 4 to 8 ohms), which usually consumes more current than the buzzer (with an input impedance of about 32 ohms.
In addition, with recent trend for reducing sizes of portable telephone sets, the loudspeakers which can be mounted on portable telephones sets usually have small output openings (mainly of 16 mm or below), and their sound pressure is usually low compared to buzzers.
Since power supply batteries are in a trend of reducing size, capacity and weight as noted above, it is extremely difficult to realize current rating reduction of the loudspeaker drive amplifier while ensuring sufficient sound pressure.